The present invention relates to an image processing apparatus and method and, more particularly, to an image processing apparatus and method for converting a binary image and outputting the converted image.
More specifically, the present invention relates to a technique for performing smoothing processing of a binary image on the basis of outline vector information extracted from an outline portion of a binary image obtained in outline smoothing processing of an original image.
The image processing technique according to the present invention can be applied to a resolution conversion technique and a bit-map image smoothing technique in OA equipment such as an electronic file apparatus, a facsimile apparatus, and the like.
The image processing technique according to the present invention can be applied to a resolution conversion technique and smoothing technique in multimedia image processing.
As a conventional resolution conversion technique of a digital binary image in, e.g., a facsimile apparatus, an SPC (Selective Processing Conversion) method, a projection method, and the like have been proposed and used.
The SPC method is a technique for attaining resolution conversion by repetitively using or periodically thinning out certain pixels of an original image a plurality of number of times corresponding to the variable magnification factor of an image (see Matsumoto & Kobayashi, "Examination of Image Quality Evaluation in Facsimile Resolution Conversion", Journal of Image Electronics, Vol. 12, No. 5, pp. 354-362, 1983).
The projection method is a technique for determining the pixel values of a converted image by projecting an original image onto a conversion image surface having a different line density, and binarizing the integrated values of corresponding pixels in the surface on the basis of the threshold value theory (see Arai & Yasuda, "Examination of Facsimile Line Density Conversion", Journal of Image Electronics, Vol. 7, No. 1, pp. 11-18, 1978).
Furthermore, as the resolution conversion and smoothing technique of a binary image, a smoothing technique which refers to a pixel pattern around the pixel of interest (Imanaka et al., "High-image Quality Conversion of Facsimile Received Image by Smoothing Processing", Preparatory Manuscript for Annual Conference of Society of Image Electronics, No. 18, 1991), or the like has been proposed, and is used in a facsimile apparatus, an electronic file, and various peripheral devices.
However, the resolution conversion method cannot avoid deterioration of image quality since a curved portion of an image becomes a stepwise (zigzag) pattern upon conversion with a large variable magnification factor. On the other hand, the resolution conversion and smoothing technique which refers to a pixel pattern around the pixel of interest can only cope with fixed variable magnification factors such as .times.2, .times.4, and the like of the pixel of interest in the main scanning and sub-scanning directions of an image.
In order to solve the above-mentioned problems, an outline smoothing method for extracting the outline of a black image (when a binary image is defined by white or black pixels) as vector data (to be referred to as "outline vector data" hereinafter) in place of directly processing a binary image in a bit-map format, and performs subsequent processing operations such as variable magnification processing, smoothing processing, and the like for the outline vector data so as to obtain a high-quality image (to be referred to as "outline processing" hereinafter), has been proposed (see Japanese Patent Application No. 3-345062).
When outline vector data is defined by the coordinate data of start and end points, the variable magnification processing can be attained in accordance with the variable magnification factor of the coordinate data. When outline vector data is defined by a difference value (length) of vectors (Japanese Patent Application No. 4-152461), the coordinate values of the origin of a vector loop can be multiplied with the variable magnification factor, and the respective vector difference values can be enlarged/reduced in accordance with the variable magnification factor. Furthermore, the smoothing processing such as isolated point/notch removal, jaggy (zigzag pattern) smoothing, and the like is attained by re-defining outline vectors by pattern matching using, as elements, the lengths and directions of outline vectors near the vector of interest with respect to the vector of interest, to be subjected to smoothing processing, of extracted vectors.
When a processing image includes a pseudo halftone image such as a picture which is not suitable for outline processing, a pseudo halftone region and a character/line image region in the image are discriminated in advance by region discrimination of a binary image, and a final image is synthesized by selecting pixels obtained by the outline smoothing method for the character/line image region, and pixels obtained by the above-mentioned SPC method for the pseudo halftone region, thereby realizing high-image quality resolution conversion.
On the other hand, a method of reducing the memory capacity required for processing and shortening the processing time by executing the outline smoothing method in units of image blocks of a predetermined amount expressed in units of lines in a binary image, e.g., in a stripe pattern (to be referred to as "stripe processing" hereinafter), has been proposed.
When the outline smoothing method is used, a resolution conversion of an image with relatively high quality, which cannot be obtained by the conventional method, can be realized within an arbitrary variable magnification factor range from a small magnification factor to a large magnification factor.
On the other hand, when the conventional outline smoothing processing is executed, a very large number of outline vectors are extracted depending on processing images, and processing such as smoothing processing, binary image reproduction processing, and the like must be performed for a large amount of outline vector data. For this reason, a work memory for storing vector data during processing must have a very large capacity, resulting in a large disadvantage in terms of cost.
Furthermore, it is difficult to integrate a large-capacity memory on an LSI. Even when a system for externally connecting a large-capacity memory to an LSI (e.g., a secondary memory such as a hard disk device) is used, the time required for memory accesses is prolonged, resulting in a low processing speed as a whole.
On the other hand, the work memory can be reduced by the method of dividing an image into stripe patterns and executing processing in units of stripes. However, this method is not necessarily satisfactory, either.